This invention relates to a process, and the apparatus for effecting such a process, for the cryogenic fractionation of gaseous hydrocarbon feeds to extract and recover the valuable heavier components thereof. The invention is particularly concerned with a process for high recovery of ethane and heavier components from a natural gas feed. The process is not limited to the recovery of paraffinic compounds such as ethane found in natural gas, but also, for example, to olefins such as ethylene often found in gases associated with petroleum refining or petrochemicals manufacture.
Conventional processes to effect very high recovery of ethane and heavier components from natural gas typically utilise a combination of heat exchange, turbo-expansion, phase separation and fractionation steps. The use of turbo-expansion produces work, which can be used to drive a compressor to supplement residual gas compression, and by removing energy from the feed gas produces low temperature.
In such conventional processes feed gas is partially condensed in a heat exchange system, which typically includes rewarming residual vapour and may include other cold streams such as refrigerant from a mechanical refrigeration cycle. Partial condensation results in a liquid stream, enriched in the valuable heavy components being recovered and a vapour stream, which may undergo further partial condensation steps. These partial condensation steps result finally in one or more liquid streams and a high pressure vapour stream. The liquid streams are expanded and fed to a demethaniser column, which removes the majority of the methane and lighter components, to produce a stable liquid stream. The high pressure vapour stream is work expanded giving a two phase stream which is fed to the demethaniser at a point above the expanded liquid streams.
It is conventional for the demethaniser column to be refluxed with a stream colder than the expander exhaust. A number of processes have been proposed, which differ in their selected demethaniser reflux stream. These processes do however share the principle of judiciously using heat exchanger surface area to make good use of the available refrigeration and to thus give lower process temperatures. Losses of the valuable ethane and heavier components in the demethaniser overheads can thus be reduced without decreasing the demethaniser column pressure and therefore without excessive power requirement.
These processes give an improvement over traditional processes, which use the expander exhaust as the top feed to the demethaniser. Increasing recovery of ethane and heavier components in these traditional processes requires a reduction in demethaniser and expander exhaust pressure to reduce temperatures. Very high ethane recovery can therefore result in uneconomically high power requirements in either recompression of the residual vapour to required product pressure, external refrigeration to increase liquids condensation or in feed gas compression which also increases liquids condensation.
It is common for the selected source of demethaniser reflux to be lean in the components being recovered. A particularly effective reflux stream is that derived from the demethaniser overheads, which in a process effecting very high recovery of ethane from natural gas may be nearly pure methane. A conventional overhead condenser, condensing overhead vapour at column pressure, can not usually be utilised due to the absence of process streams at a lower temperature to provide the necessary refrigeration. In the process of U.S. Pat. No. 4,889,545, a portion of the demethaniser overhead vapour is compressed in a standalone compressor, such that it can be condensed in heat exchange with other process streams to reflux the demethaniser.
U.S. Pat. Nos. 4,171,964 and 4,157,904 describe processes in which streams relatively rich in ethane are sent to the top of the demethaniser to act as reflux, and thus do not provide very high recovery of ethane. GB 2,309,072 and WO 98/50742 disclose hydrocarbon gas processing apparatus wherein a recycle stream is used to reflux the demethaniser.
A configuration in which a portion of the residue gas, which has been rewarmed and compressed to a pressure suitable for export, is recycled, condensed, subcooled and expanded to reflux the demethaniser column shown in FIG. 1. This configuration is less thermodynamically efficient than that of a standalone compressor, due to the losses inherent in warming and re-cooling the residue vapour. The process is however simpler as a standalone compressor is not required.
The pressure at which the recycle stream is cooled and condensed will typically be optimised to minimise residual gas compression power requirement. It is desirable to sub-cool the recycle stream to within a small approach to the demethaniser overhead temperature, which is the coldest stream in the process. This minimises evolution of vapour on expanding the liquid to column pressure and therefore maximises the liquid available to reflux the rising vapour in the column. At lower recycle stream pressures, compression power requirements are reduced, but the cooling curve becomes less linear and a pinch can occur which limits the temperature to which the recycle stream can be cooled.
According to one aspect of the invention there is provided a process for the separation of a heavier hydrocarbon fraction from a gaseous feed comprising a mixture of hydrocarbons, which process comprises:
(a) cooling the gaseous feed to produce a partially condensed stream
(b) separating the partially condensed stream to form a first liquid stream and a first gaseous stream
(c) subcooling at least a portion of the first liquid stream
(d) expanding said subcooled stream
(e) passing at least a portion of the expanded stream from (d) as a liquid feed to a fractionation column
(f) producing a cooled stream by cooling a separated lights fraction of the feed
(g) recovering said heavier hydrocarbon fraction as a bottoms fraction from said column
characterised in that (i) at least a part of the subcooling required in subcooling step (c) is provided by transfer of heat to the expanded stream from (d), (ii) at least a portion of the cold required to cool the separated lights fraction to produce said cooled stream (f) is provided by transfer of heat to the expanded stream from (d), and (iii) at least a portion of said cooled stream (f) is introduced to an upper part of the column.
It will be appreciated that as a result of having been expanded in step (d) and subjected to a heat transfer operation to provide at least part of the subcooling required from step (c), the feed to the fractionation column will be in an at least partially evaporated state.
In a preferred manner of operation according to the invention at least a part of the subcooling required in subcooling step (c) is provided by transfer of heat to overhead vapour from said fractionation column. When operated in this manner overhead vapour from the fractionation column may be heated in heat exchange with the first liquid stream from step (b) prior to expansion. More preferably a process is provided wherein overhead vapour from the fractionation column and the expanded stream from step (d) are heated in heat exchange with said first liquid stream from step (b) prior to expansion in a single heat exchanger.
It will be appreciated that in accordance with the invention, net refrigeration available within the heat exchanger may be used to cool said stream (f). Preferably the cooled stream (f) comprises a recycle stream derived from overheads from the fractionation column, and is used to reflux the top section of the fractionation column.
By heat-exchanging the first liquid stream and the separated lights fraction with the expanded stream from (d), against the evaporating stream derived from the subcooled stream, a stream of lower temperature is produced that ultimately leads to reduced overheads temperature and increased recovery.